As you can see in the strace output ffmpeg searches for a .ffpreset file instead of .avpreset that is the presets file extension used in libav-tools (all the preset files under /usr/share/avconv/ are .avpreset)

Just as a temporary workaround simply adding a symlink (Eg: ln -s /usr/share/avconv/libx264-fast.avpreset /usr/share/avconv/libx264-fast.ffpreset) do the trick.